Intelligent lighting for the 21st century | Part 3

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Learn more about two great innovative camera features

Ultra-high resolution for industrial imaging

Many industrial inspection applications such as flat panel display inspection,
PCB inspection and document scanning require ultra high resolution imaging to
allow the required level of detail to be produced in the image.

Flat panel displays (FPDs) cover a growing number of technologies including
OLEDs (organic light emitting diodes) and LCDs (liquid crystal displays). Each
new generation of FPDs has increasing pixel densities requiring even higher
resolution and sensitivity for quality inspection combined with pressure not
to increase inspection times. Document scanning applications include
continuous verification and/or quality inspection of numbered print and
inspection of symbols and labels on web, sheet or single documents, as well as
inspection of security features by checking the presence, position and
integrity of applied features such as such as foil and hologram devices and
base paper inserts like security threads.

Camera sensors with more and more pixels

These high resolution inspection applications generally utilise dedicated
inspection equipment complete with fully integrated cameras, optics and
illumination systems. The majority of imaging applications utilise area scan
cameras and recent advances in CMOS and CCD technology have resulted in the
availability of camera sensors with more and more pixels, providing ever
increasing spatial resolution while maintaining or improving the camera frame
rate. However, the number of available pixels is not the sole consideration.
Mass-produced cameras sensors for smart phones typically can offer resolution
of the order of 12 - 16 megapixels (Mpixels), yet the sensors themselves are
very small and are very much cost driven. This means that they have very small
pixels leading to poor light collection and poor signal-to-noise, which alone
would preclude them from being used in industrial imaging applications.
However they also have limited on chip exposure control resulting in a rolling
shutter system where all the pixels are not exposed at the same time.

Requirements for industrial imaging cameras

While this might not be noticed in consumer ‘happy snapping’ applications,
there are a host of other requirements for industrial imaging cameras in
addition to the number of pixels. These include pixel size & shape,
sensitivity, frame rate, full image exposure control and triggering
characteristics, dynamic range, spectral response, image pre-processing,
partial scanning, advanced multiple region readout and sequence control among
others. In addition cameras are characterised by the interfacing standard that
they employ to transfer data to the host computer for measurement and analysis
(e.g. CameraLink, GigE Vision, USB3 Vision, CoaXPress, CameraLink HS etc).
While to the novice the significant price difference between camera phones and
machine vision high resolution cameras seems unfathomable, the significantly
lower manufacturing quantities and larger sensor sizes for machine vision mean
a completely different pricing model.

How many pixels are enough?

New industrial vision cameras come to market on a regular basis and some
recently announced high resolution area scan cameras include the 29 Mpixel AV
Prosilica GT6600 and the Thermoelectric Peltier Cooled Vieworks VP-29MC-M/C 5,
both of which have sensors with 6576 x 4384 pixels while the new JAI Spark SP-
20000C-PMCL is a 20 MPixel camera providing 5120 x 3840 pixels. But what if
you have an application that requires more pixels?

Different solutions for applications requiring more pixels

One solution would be to use multiple cameras to produce a number of images
that are stitched together, or to keep to a single camera and move either the
sample or the camera to produce a number of images that can be stitched
together to produce a composite image. A third option is to use pixel shift
technology which will be discussed in detail in the next section. A different
approach is the use of linescan cameras. Line scan cameras, in their simplest
forms, use a sensor with a single line of pixels. Typical resolutions vary
from 512 x 1 pixels up to 16384 x 1 pixels. Line scan cameras work by building
up an ‘area image’ (typically in a frame grabber) from multiple lines by
moving either the sample or the camera. As only the width of the resulting
image is fixed, it not only allows a very high resolution image to be created
but also means that the aspect ratio of the image can be chosen to match the
particular sample. Typically area scan sensors have an aspect ratio of 4:3,
16:9 or 1:1, but line scan images are not limited to this. Even with a 4:3
ratio a 16k line scan camera would produce either a 358 Mpixel or 201 Mpixel
image, depending on the orientation.

Considerations in the use of line scan cameras

Two key considerations in the use of line scan cameras (more about line scan technology) are movement and
alignment. Relative movement between the sample and the camera is essential to
allow the image to be produced and the rate of movement must be appropriate
for the particular imaging requirement and must have no variation otherwise
distortion will appear in the image. Similarly, alignment of the camera is
crucial since If the line scan camera’s line sensor is not perpendicular to
the direction of movement, the resulting image will contain skew.

A recent development using the line scan approach is the introduction of
contact image sensors. These behave in the same way as fax machines and
scanners but on an industrial basis with far higher resolutions, speeds and
interfacing. The advantage is reduction in working distance, removal of lens
distortion and easier set up.

Pixel shift technology

The use of pixel shift technology, however, significantly extends the
resolution capability of area scan sensors. The Vieworks VNP-29MC CameraLink
industrial vision cameras feature a 29 Mpixel resolution (6576 x 4384 pixels)
CCD sensor as standard, but also have pixel shift technology which can provide
an extended resolution of 260 million pixels (19728 × 13152) for ultra high
resolution applications. The sensor is mounted on a precise piezoelectric
crystal stage which allows the CCD to be nano-shifted by 1/3 or 1/2 of a
pixel. This enables the standard resolution to be extended by 4 times to 115
million pixels or by 9 times to 260 million pixels.

The pixel shifting process

The pixel shifting process is shown in Figure 1. In this example, the sensor
is shifted precisely by ½ pixel in the X and Y directions as shown, with the
resulting image being a combination of the 4 individual images giving improved
resolution (4 shot result image) in comparison with the standard output image
(1 shot result image). Image combination is carried out in software on the
processing PC. A similar procedure can be carried out by shifting the sensor
by 1/3 pixel to produce 9 resultant images. The camera is available in both
monochrome and colour versions and the pixel shifting process has additional
benefits for colour imaging. The colour CCD camera uses Bayer Interpolation to
produce colour images and unwanted artifacts can occur such as colour moiré or
false color pixels.

Figure 1: Principle of pixel shifting technology

Ideally suited for low light/contrast industrial applications

Using pixel shifting, however, no colour artifacts or aliasing will occur and
the colour resolution is optimized, as shown in Figure 2. The VNP-29MC sensors
can be thermo-electric Peltier (TEC) cooled to as much as 15 degrees below
ambient for improved sensitivity. This provides extremely stable operating
conditions, reducing noise and enabling exposure for a long period of time or
at higher gain levels to increase camera sensitivity. This makes the cameras
ideally suited to low light or low contrast level or non-uniform brightness
industrial applications.

Figure 2: Optimisation of colour using pixel shifting

Improved colour resolution and larger field of view

The improvement in resolution is illustrated on a section of banknote in
Figures 3 and 4. Figure 3 is recorded at the standard resolution settings, but
the pattern is pixelated and unclear. Figure 4 shows the same region using
pixel shift technology and the pattern is clearly resolved. In addition to the
obvious benefits of improved resolution, at any given resolution setting, the
pixel shift gives a larger field of view if fitted with the appropriate lens,
which means it could avoid the need to stitch multiple images together or remove the need for multiple cameras, saving both time and money.